The invention relates to an optical scanning apparatus for scanning objects with a switchable resolution. The invention also relates to a circuit for an optical sensor with a switchable resolution.
Optical Scanning devices are used to make an electronic image of an object, for example, in the form of digital data. Such an optical scanning device might be, for example, a scanner for a photocopying machine or a fax machine, a so-called scanner to plug to a computer system for scanning papers and photographs or a telecine for scanning images in a film.
One of the most important performance parameters is the resolution of the scanning system. For telecines high-resolution systems become ever more important, especially in high-end postproduction applications which use the scanned data for sizing, zooming, multilayering in High Definition TV or data application.
The front end of a telecine is the first and most important subsystem, which is used to convert the optical information contained in the film into electric signals that can be processed by various signal processors. As a key element, the optical and electrical quality of this scanning system determines the maximum available quality of the electrical output, either digital or analog, TV standard or data format.
One restriction of currently available telecines is scanning speed versus scanned resolution. Standard resolution systems, such as the Quadra telecine, a product of Philips Digital Video systems, uses lower resolution scanners in order to provide speeds of up to 30 frames/sec., while high-resolution systems often operate at speeds down to one frame/sec. or even less.
Depending on the chosen technique, telecines utilize different types of scanning devices. A preferred scanning device uses an optical sensor built up of charge-coupled devices. There are charge-coupled devices which transfer the charge of xe2x80x98oddxe2x80x99 and, xe2x80x98evenxe2x80x99 pixels to two different shift registers. As both shift registers can operate at half the pixel rate, the processing clock frequencies and the bandwidth of the analog processing can be halved, compared to common charge-coupled devices which have only a single line shift register.
It is an object of the invention to provide a given resolution at a maximum scanning speed.
This object is achieved by using summing means for eventually summing the contents of two adjacent pixels.
In a low-resolution mode every two adjacent pixels are added together. Therefore, the sum has a better signal-to-noise ratio and can be processed twice as fast without losing signal quality. This means that with up-to-date circuits, real-time scanning with 30 frames/sec. can be performed. Due to the worse signal-to-noise ratio in high-resolution mode, where each pixel has to be processed separately without being summed, the scanning speed has to be halved. Neither the less low-resolution scanning nor high-resolution scanning can be carried out by the same sensor. This helps to minimize costs.
Many high-resolution applications, such as data transfer and processing with a workstation, do not need the speed of 30 frames/sec., while lower resolution applications need the scanning in real-time mode. The described arrangement gives both high-speed ( less than 30 frames/sec.) standard resolution and lower speed ( less than 15 frames/sec.) high-resolution scanning.